In recent years, the users have required image forming apparatuses, employing the electro-photographic method, which form color images, exhibiting high quality images, comparative to offset prints.
To form multi-color image by the electro-photographic method, various types of methods can be listed, one of which superimposes each color toner image on an intermediate transfer body.
In a case that an intermediate transfer body is used, mostly used is a method in which when the toner image is to be transferred onto the intermediate transfer body from the photoconductor, a first transfer section is formed, wherein a transfer roller carrying an electrical voltage is controlled to press the back of the intermediate transfer body against the photoconductor.
However, during the above transfer operation, some portions of the toner image may still remain on the photoconductor, and are not transferred, whereby desired transfer operation is not conducted, which is a problematic matter for the image forming apparatus. Specifically, during the image transfer operation, toner particles, which form a line drawing or a central area of dotted images, receive high pressure by being nipped between the photoconductor and the intermediate transfer body, so that the toner particles tend to agglutinate, and the agglutinated toner particles are not transferred to the intermediate transfer body, but remain on the photoconductor. Accordingly the middle portion of the image remain blank, which phenomenon is usually regarded as “the hollow defect”.
Said hollow defect occurs due to the magnitude relationship between the adhering force between the toner particles and the photoconductor, and the adhering force between the toner and the intermediate transfer body. If the adhering force between the toner particles and the photoconductor is reduced, the probability of occurring of said hollow defect can be minimized.
An effective method to reduce the adhering force between the toner particles and the photoconductor is one in which a lubricant is applied onto the surface of the photoconductor. A metal stearate, such as zinc stearate or the like, is generally used as the lubricant. As the method to apply the lubricant, a rotating brush scrapes off some solid lubricant, at a position distant from the position where the rotating brush later touches the photoconductor, and the solid lubricant carried by the rotating brush is applied onto the surface of the photoconductor (see FIG. 2). Further, if the friction factor between photoconductor 10 and blade 15 is relatively great, photoconductor 10 and blade 15 are deteriorated due to the friction. However, cleaning performance of cleaning device 14 can be increased due to the lubricant applied onto the surface of photoconductor 10.
Concerning the conventional arts, Unexamined Japanese Patent Application Publication No. 8-137,354 discloses that the lubricant is applied onto a cleaning brush through a flicker roller in a cleaning device, whereby in order to prevent a filming layer or fine powder among the toner particles, which generates black spots on images, from adhering onto the photoconductor, further, in order to prevent the toner particles adhered onto background areas from increasing, and still further, in order to obtain a high quality image exhibiting a high adhering amount of the image characters, fog density of the toner on the photoconductor is detected so that an applied amount of lubricant is appropriately controlled.
Further, Unexamined Japanese Patent Application Publication No. 2002-244,486 discloses a technology in which for an image forming apparatus, employing toner particles with a small diameter, or employing minute toner particles, solid lubricant is applied onto the surface of a photoconductor drum by a rotating brush, so that the surface of the photoconductor is coated with a thin coat of the lubricant, whereby the amount of lubricant to be applied onto the surface of the photoconductor is controlled based on information of outputting images which were previously stored.
In the normal state, since an appropriate amount of lubricant is applied onto the surface of the photoconductor at the cleaning device, the electrical potential is controlled within a certain range. However, if images carrying relatively high text ratio, are continuously printed, a large amount of toner is supplied to a brush, whereby the amount of toner accumulated on the brush tends to become greater than the amount of toner removed by the flicker. Due to this situation, the brush cannot remove the appropriate amount of the lubricant, so that the surface of the photoconductor cannot be coated with the appropriate amount of lubricant. Further, since the toner particles have been accumulated onto the brush, when the brush contacts the surface of the photoconductor, any portion on which the toner particles are directly adhered, becomes to exist on the photoconductor, and any portion which the brush directly contacts also becomes to exist on the photoconductor. If the brush directly applies frictional electrification onto the photoconductor, not through the lubricant, any one of the photoconductor or the brush is charged to be a negative electrode or a positive electrode, and vise versa, due to the frictional electrification, which depends on the constituent materials of the photoconductor and the brush. Still further, if the toner accumulated on the brush contacts the photoconductor due to the frictional electrification, the same phenomenon as above occurs. Due to the frictional electrification, the surface of the photoconductor is highly charged to become the negative electrode or positive electrode.
Under the above-described state, if the amount of lubricant applied onto the photoconductor is measured by the detection of the fog density of toner on the photoconductor, it is difficult to measure the amount of lubricant with high accuracy, because the measurement depends upon the using condition of the developer, the thickness of the surface of the photoconductor, and the ambient environment of the image forming apparatus. Further, if the amount of lubricant applied onto the photoconductor is measured based on image forming information such as the printing pattern, it is also difficult to measure the amount of lubricant with high accuracy, due to insufficient information.